I. Field of the Invention
The present invention relates to communications systems. More particularly, the present invention relates to a novel and improved method and apparatus for maximizing total average service quality to users in a multi-user communication system by controlling the data transmission rates to and from users of the multi-user communication system.
II. Description of the Related Art
The term “multiple access” refers to the sharing of a fixed communications resource by a plurality of users. A typical example of such a fixed communications resource is bandwidth. There are three basic ways to increase the throughput or data rate of an individual user accessing a communications resource. The first way is to increase the transmitters radiated power or alternatively to reduce system losses so that the received signal to noise ratio (SNR) is increased. The second way is to increase the allocation of bandwidth to the user. The third approach is to make allocation of the communications resource more efficient.
Some of the more common methods of providing multiple access to a communications resource involve both analog and digital communication modulation schemes. Such schemes include frequency division, time division and spread spectrum techniques. In frequency division multiple access (FDMA) techniques, each user is allocated one or more specific sub-bands of frequency. In time division multiple access (TDMA) techniques, periodically recurring time slots are identified, and for each segment of time each user is allocated one or more time slots. In some TDMA systems, users are provided a fixed assignment in time, and in other systems users may access the resource at random times. In spread spectrum communications, users share a common frequency band. Using frequency hopping (FH) modulation, the signal is modulated upon a carrier which changes in frequency according to a predetermined plan. In direct sequence (DS) modulation, the user signal is modulated with a pseudorandom code. In one type of code division multiple access (CDMA) technique which uses direct sequence spread spectrum modulation, a set of orthogonal or nearly orthogonal spread spectrum codes (each using full channel bandwidth) are identified, and each user is allocated one or more specified codes.
In all multiple access schemes, a plurality of users shares a communications resource without creating unmanageable interference to each other in the detection process. The allowable limit of such interference is defined to be the maximum amount of interference such that the resulting transmission quality is still above a predetermined acceptable level. In digital transmission schemes, the quality is often measured by the bit error rate (BER) or frame error rate (FER). In digital speech communications systems, the overall speech quality is limited by data rate allowed for each user, and by the BER or FER.
Systems have been developed to minimize the data rate required for a speech signal while still providing an acceptable level of speech quality. If speech is transmitted by simply sampling and digitizing the analog speech signal, a data rate on the order of 64 kilobits per second (Kbps) is required to achieve a speech quality equivalent to that of a conventional analog telephone. However, through the use of speech analysis, followed by the appropriate coding, transmission, and resynthesis at the receiver, a significant reduction in the data rate can be achieved with a minimal decrease in quality.
Devices which employ techniques to compress speech by extracting parameters that relate to a model of human speech generation are typically called vocoders. Such devices are composed of an encoder, which analyzes the incoming speech to extract the relevant parameters, and a decoder, which resynthesizes the speech using the parameters which are received from the encoder over the transmission channel. As the speech changes, new model parameters are determined and transmitted over the communications channel. The speech is typically segmented into blocks of time, or analysis frames, during which the parameters are calculated. The parameters are then updated for each new frame.
A more preferred technique to accomplish data compression, so as to result in a reduction of information that needs to be sent, is to perform variable rate vocoding. An example of variable rate vocoding is detailed in U.S. Pat. No. 5,414,796 entitled “Variable Rate Vocoder,” assigned to the assignee of the present invention and incorporated herein by reference. Since speech inherently contains periods of silence, i.e. pauses, the amount of data required to represent these periods can be reduced. Variable rate vocoding most effectively exploits this fact by reducing the data rate for these periods of silence. A reduction in the data rate, as opposed to a complete halt in data transmission, for periods of silence overcomes the problems associated with voice activity gating while facilitating a reduction in transmitted information, thus reducing the overall interference in a multiple access communication system.
It is the objective of the present invention to modify the variability of the transmission rate of variable rate vocoders, and any other variable rate data source, in order to maximize utilization of the communications resource.